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Cerebellum: Gross anatomy and main connections

This
note describes the gross features of cerebellum and its main connections.

Developmental source: It develops from the metencephalon division of rhombelcephalon (hind brain).

Location – posterior
cranial fossa

Separated from
the cerebrum by a fold of dura mater called tentorium cerebelli

Separated from
pons and medulla by the cavity of fourth ventricle

Presenting
parts

Vermis – near the
midline

Two lateral hemisphere

Two surfaces: superior and inferiorsurfaces
separated by horizontal fissure

On the inferior
aspect, the two hemispheres are separated by a depression called the valeculla.

On each side the
vermis is separated from the corresponding cerebellar hemisphere by a paramedian sulcus

Anteriorly and
posteriorly, the hemisphere extends beyond the vermis and are separated by anterior and posterior cerebellar notch.

The surface of
the cerebellum is marked by a series of fissures
that run more or less parallel to one another. The fissures subdivide the
surface of the cerebellum into narrow leaf like bands or folia.

Further, these
fissures divide the cerebellum into lobes and lobules.

The deepest
fissures in the cerebellum are

The primary fissure – running transversely
across the superior surface

Posterolateral
fissure –
present in the inferior aspect

Lobes:

Anterior
lobe
– the part anterior to the primary fissure

Posterior
lobe
– the part between the two fissures (also called the middle lobe)

The remaining
part is the flocculonodular lobe.

The vermis also
consists of (from before backward)

Lingual, central
lobule and culmen (in the anterior lobe)

Declive, folium,
tuber, pyramid and uvula (in the middle lobe)

Nodule (in
flocculonodular lobe)

Except lingula,
each subdivision of vermis is laterally related to a part of the hemisphere.

In the anterior
lobe,

Central lobule is
related to Ala

Culmen is
related to anterior quadrangular lobule

In the middle
lobe

Delive is
related to posterior quadrangular lobule

Folium is
related to superior semilunar lobule

Tuber is related
to inferior semilunar lobule and gracile

Pyramid is
related to biventral lobule and

Uvula is related
to tonsil

Nodule is
laterally continuous with the flocculus.

Grey
matter and deep nuclei of Cerebellum

Most of the grey
matter of the cerebellum is arranged as a thin layer covering the central core
of white matter. Embedded within the white matter are masses of grey matter
called deep cerebellar nuclei and they are:

Dentate nucleus
– lies in the center of each cerebellar hemisphere

Emboliform
nucleus – lies on the medial side of the dentate nucleus

Globose nucleus – lies medial to the emboliform nucleus

Fastigial
nucleus– lies close to the midline

White
matter of the cerebellum

The central core
of each cerebellar hemisphere is made up of white matter which is continuous
with the cerebellar peduncles. The two sides of the white matter is connected
by a thin lamina of fibers that forms the roof of the fourth ventricle. The
upper part of this lamina forms the superior medullary velum and lower part
forms the inferior medullary velum.

The white matter
consists of:

1.Afferent
fibers entering the cerebellum from outside

2.Projection
fibers from the cerebellar cortex to the cerebellar nuclei

3.Association
fibers interconnecting different parts of the cerebellar cortex

4.Commissural
fibers connecting the two cerebellar hemisphere

5.Fibers
from the cerebellar nuclei to centers outside the cerebellum

Cerebellar
peduncles:
the fibers entering or leaving the cerebellum pass through three thick bundles
known as cerebellar peduncles : superior middle and inferior

Superior
cerebellar peduncle

Consists of the
fibers mainly arising in the cerebellar nuclei (mainly dentate nucleus) that
enter the midbrain and cross to the opposite side before ending in the red
nucleus (mainly). Many of the fibers also ascend to the thalamus.

The right and
left superior peduncles are connected to each other by a thin lamina of white
matter called the superior or anterior medullary velum.

Middle
cerebellar peduncle

It begins as a
lateral continuation of the ventral part of the pons. The fibers in this
peduncle arise from the pontine nuclei which cross to the opposite side

Inferior
Cerebellar peduncle

Also called the
restiform body, it is a thick bundle of fibers that connect the posterolateral
part of the medulla with the cerebellum.

Main
connection of cerebellum

The fundamental points to be considered regarding the
connections of the cerebellum are

1.Afferent fibers terminate in the
cortex

2.Efferent fibers arising in the
cortex end in the cerebellar nuclei and

3.Fibers arising in the nuclei project
to centers outside the cerebellum

The cerebral cortex sends information to the cerebellum by
three pathways:

(1) the corticopontocerebellar pathway,

(2) the cerebro-olivocerebellar pathway, and

(3) the cerebroreticulocerebellar pathway.

Corticopontocerebellar Pathway

The corticopontine fibers arise from nerve cells in the
frontal, parietal, temporal, and occipital lobes of the cerebral cortex and
project to the pontine nuclei after descending through the corona radiata and
internal capsule.The pontine nuclei give rise to the transverse fibers of the
pons, which cross the midline and enter the opposite cerebellar hemisphere
as the middle cerebellar peduncle.

Cerebro-olivocerebellar Pathway

The cortico-olivary fibers arise from nerve cells in the
frontal, parietal, temporal, and occipital lobes of the cerebral cortex to
terminate bilaterally on the inferior olivary nuclei after passing through the
corona radiata and internal capsule. The inferior olivary nuclei give rise to
fibers that cross the midline and enter the opposite cerebellar hemisphere
through the inferior cerebellar peduncle.

Cerebroreticulocerebellar Pathway

The corticoreticular fibers arise from nerve cells from many
areas of the cerebral cortex, particularly the sensorimotor areas. They descend
to terminate in the reticular formation on the same side and on the opposite
side in the pons and medulla. The cells in the reticular formation give rise to
the reticulocerebellar fibers that enter the cerebellar hemisphere on the same
side through the inferior and middle cerebellar peduncles.

Cerebellar Afferent Fibers From the
Spinal Cord

The spinal cord sends information to the cerebellum from
somatosensory receptors by three pathways:

(1) the anterior spinocerebellar tract,

(2) the posterior spinocerebellar tract, and

(3) the cuneocerebellar tract

Anterior Spinocerebellar Tract

The axons entering the spinal cord from the posterior root
ganglion terminate by synapsing with the neurons in the nucleus dorsalis
(Clarke's column) at the base of the posterior gray column. Most of the axons
of these neurons cross to the opposite side and ascend as the anterior
spinocerebellar tract in the contralateral white column. The fibers enter the
cerebellum through the superior cerebellar peduncle and terminate as mossy
fibers in the cerebellar cortex. It is believed that those fibers that cross
over to the opposite side in the spinal cord cross back within the cerebellum.

The anterior
spinocerebellar tract fibers convey muscle joint information from the muscle
spindles, tendon organs, and joint receptors of the upper and lower limbs. It
is also believed that the cerebellum receives information from the skin and
superficial fascia by this tract.

Posterior Spinocerebellar Tract

The axons entering the spinal cord from the posterior root
ganglion enter the posterior gray column and terminate by synapsing
on the neurons at the base of the posterior gray column. The axons of these
neurons enter the posterolateral part of the lateral white column on the same
side and ascend as the posterior spinocerebellar tract to the medulla oblongata.
Here, the tract enters the cerebellum through the inferior cerebellar peduncle
and terminates in the cerebellar cortex. The posterior spinocerebellar tract
receives muscle joint information from the muscle spindles, tendon organs, and
joint receptors of the trunk and lower limbs.

Cuneocerebellar Tract

These fibers originate in the nucleus cuneatus of the
medulla oblongata and enter the cerebellar hemisphere on the same side through
the inferior cerebellar peduncle. The cuneocerebellar tract receives muscle
joint information from the muscle spindles, tendon organs, and joint receptors
of the upper limb and upper part of the thorax.

Cerebellar Afferent Fibers From the
Vestibular Nerve

The vestibular nerve receives information from the inner ear
concerning motion from the semicircular canals and position relative to gravity
from the utricle and saccule. The vestibular nerve sends many afferent fibers directly
to the cerebellum through the inferior cerebellar peduncle on the same side.
Other vestibular afferent fibers pass first to the vestibular nuclei in the
brainstem, where they synapse and are relayed to the cerebellum. They enter the
cerebellum through the inferior cerebellar peduncle on the same side.

Other Afferent Fibers

In addition, the cerebellum receives small bundles of
afferent fibers from the red nucleus (rubrocerebella tract) and the tectum
(tectocerebellar tract).

Cerebellar Efferent Fibers

The entire efferent outflow of the cerebellar cortex is
through the axons of the Purkinje cells via deep cerebellar nuclei. A few
Purkinje cell axons pass directly out of the cerebellum to the lateral
vestibular nucleus.

The efferent fibers from the cerebellum connect with the red
nucleus, thalamus, vestibular complex, and reticular formation.

Globose-Emboliform-Rubral Pathway

Axons of neurons in the globose and emboliform nuclei travel
through the superior cerebellar peduncle and cross the midline to the opposite
side in the decussation of the superior cerebellar peduncles. The fibers end by
synapsing with cells of the contralateral red nucleus, which give rise to axons
of the rubrospinal tract. Thus, this pathway crosses twice, once in the decussation
of the superior cerebellar peduncle and again in the rubrospinal tract close to
its origin. By this means, the globose and emboliform nuclei influence motor
activity on the same side of the body.

Dentothalamic Pathway

Axons of neurons in the dentate nucleus travel through the
superior cerebellar peduncle and cross the midline to the opposite side in the decussation
of the superior cerebellar peduncle. The fibers end by synapsing with cells in
the contralateral ventrolateral nucleus of the thalamus. The axons of the
thalamic neurons ascend through the internal capsule and corona radiata and
terminate in the primary motor area of the cerebral cortex. By this pathway,
the dentate nucleus can influence motor activity by acting on the motor neurons
of the opposite cerebral cortex; impulses from the motor cortex are transmitted
to spinal segmental levels through the corticospinal tract. Since
most of the fibers of the corticospinal tract cross to the opposite side in the
decussation of the pyramids or later at the spinal segmental levels, the
dentate nucleus is able to coordinate muscle activity on the same side of the
body.

Fastigial Vestibular Pathway

The axons of neurons in the fastigial nucleus travel through
the inferior cerebellar peduncle and end by projecting on the neurons of the lateral
vestibular nucleus on both sides. Some Purkinje cell axons project directly to
the lateral vestibular nucleus. The neurons of the lateral vestibular nucleus
form the vestibulospinal tract. The fastigial nucleus exerts a facilitatory
influence mainly on the ipsilateral extensor muscle tone.

Fastigial Reticular Pathway

The axons of neurons in the fastigial nucleus travel through
the inferior cerebellar peduncle and end by synapsing with neurons of the
reticular formation. Axons of these neurons influence spinal segmental motor
activity through the reticulospinal tract.

Test Yourself

1)What is the developmental source of cerebellum?

a)Telencephalon

b)Diencephalon

c)Metencephalon

d)Myelencephalon

2)Which of the following structures connect the
cerebellum with the mid brain?

a)Superior
cerebellar peduncle

b)Middle
cerebellar peduncle

c)Inferior
cerebellar peduncle

d)Crus
cerebri

3)Which of the following structures connect the
cerebellum with the pons?

a)Superior
cerebellar peduncle

b)Middle
cerebellar peduncle

c)Inferior
cerebellar peduncle

d)Cerebral
aqueduct

4)Which of the following structures connect the
cerebellum with the medulla oblongata?

a)Superior
cerebellar peduncle

b)Middle
cerebellar peduncle

c)Inferior
cerebellar peduncle

d)Stria
medullaris

5)Which of the following cavity is found in relation
to cerebellum?

a)Lateral
ventricles

b)Third
ventricle

c)Cerebral
aqueduct

d)Fourth
ventricle

6)Which
of the following structures separate the cerebellum from cerebrum?